The present invention relates to the measuring of blood oxygen saturation, and in particular to validating the accuracy of measured oxygen saturation values.
Pulse oximetry is commonly used for measuring and displaying various arterial blood characteristics including blood oxygen saturation of hemoglobin in arterial blood, the pulse rate as the rate of blood pulsation in the arteries corresponding to the heart rate of the patient, or a perfusion indicator. Pulse oximetry represents a well-established technique in the art and needs only to be briefly discussed herein.
Pulse oximeters generally determine the arterial oxygen saturation of hemoglobin (also called SpO2 or SaO2 measurement) by way of a non-invasive technique using two different monochromatic light sources typically formed by light emitting diodes (LEDs). An example for a pulse oximeter is the Hewlett Packard Component Monitoring System with the Pulse Oximeter Module, the `HP M1020A`.
As known in the art of pulse oximetry, the light of both light sources is attenuated by static and dynamic absorbers on its path through the patient's body to a light detector. The arterial blood whose quantity varies with the time synchronously with the patient's heartbeat represents the only dynamic absorber during the pulse period. All other absorbers, such as skin, tissue or bone, are not time-variant. Thus, pulse oximeters make use of the pulsatile component of arterial blood generated by the heartbeat at only two spectral lines.
The light detector receives the modulated light intensities of each wavelength. The signals are usually amplified, low pass filtered, converted from analog to digital and further processed. A pulse finding algorithm analyses the received signals, which are so-called spectrophotometric signals, for identifying the pulses and for determining the pulse. After identifying the pulse period, the diastolic and systolic values of the spectrophotometric signals are determined and the so-called relative absorption ratios are derived therefrom. Subsequently, in a saturation calculation algorithm the arterial oxygen saturation is computed from the relative absorption ratio using calibration data and so-called extinction coefficients from the absorption spectrum of hemoglobin and oxyhemoglobin at the appropriate wavelengths. The mathematical background therefor, which makes use of Lambert-Beer's law, has been described in sufficient detail in a multiplicity of former publications such as EP-A-262 778.
In parallel to the calculation of the oxygen saturation, the period between pulses is converted into the beat-to-beat pulse rate (rate=1/period). The beat-to-beat pulse rates are then averaged over a certain intervals or number of beats to generate a more or less stable value of the pulse rate. Typical averaging is done over 4,8 or 18 beats, or over 5 to 20 seconds.
Since the early 1980s, when pulse oximetry was introduced, this non-invasive method of monitoring the arterial oxygen saturation level in a patient's blood has become a standard method in the clinical environment because of its simple application and the high value of the information applicable to nurses and doctors. It has become as common in patient monitoring to measure the oxygen level in the blood as to monitor heart activity with the ECG. In some application areas, like anesthesia in a surgical procedure, it is mandatory for doctors to measure this vital parameter.
More background information about pulse oximetry is given e.g. by S. Kastle et al., "A New Family of Sensors for Pulse Oximetry", Hewlett-Packard Journal, February 1997, pages 39-53.
U.S. Pat. No. 4,928,692 (Goodman) discloses a method for synchronizing the sampling of a signal that is then to be processed by a pulse oximeter. The described technique acts as a filter that gates the input signal for further processing in the pulse oximeter. The method is based on the real time ECG-Signal (and only on this one) with its known QRS shape characteristic as the gating trigger.
Pulse oximetry, however, relies on the fact that the arterial blood is the only pulsating component that causes a pulsatile change of the light absorption used to determine the oxygen saturation. When the source of the pulsatile component is not the patient's arterial blood flow, the oxygen saturation measurement, however, might derive inaccurate values. In case of standard pulse oximetry (e.g. adult, pediatric, neonatal) motion artifacts can cause other non-arterial pulsating components. In case of e.g. fetal pulse oximetry using reflectance sensors, the sensor can accidentally pick up the mother's pulsating blood instead of the fetal pulsating blood and lead to a wrong value of the oxygen saturation. In general, oxygen saturation values derived by pulse oximetry might not be sufficient accurate due to a strong impact of pulsatile sources other than the patient's arterial blood flow.
It is therefore an object of the present invention to provide an improved pulse oximetry.